The invention provides methods of threshold addressing of electrophoretic displays. More particularly, the invention relates to materials and methods for providing a particle-filled capsule in which the capsules are addressed with two voltages or, alternatively, at a threshold.
In an electrophoretic display (encapsulated or not), the particles contained within the display move by being driven according to an applied electric field. This movement produces a display state with the particles in one location and a display state with particles in a second location. However, in a conventional electrophoretic display, each pixel must be addressed individually to cause the display to operate. The provision of address lines that address individual pixels at predetermined locations in a display can be quite complex and expensive. In color displays, the necessity to address number of color elements per pixel increases the complexity and expense of manufacture of such displays.
The bistable nature of electrophoretic displays is another beneficial feature of these displays as compared to many other types of displays. Electrophoretic displays can retain their state for some period of time without the active application of power or of a driving signal, as compared to many other types of active displays such as cathode ray tubes, LEDs, LCDs and the like, which lose their display as soon as either (or both) of power or a driving signal is removed. Thermal or gravitational forces ultimately can degrade the display state of an electrophoretic display by inducing undesired motion in the particles that comprise such a display. Electrophoretic displays in general can benefit from an increased duration of a stable display state.
The present invention relates to methods and materials for an electrophoretic display such that the particle motion is a controlled, non-linear function of the applied electric field. More particularly, the techniques and materials of the invention prevent, or at least significantly impede, the motion of charged pigment particles in the display under one set of conditions, while allowing them to move relatively freely under a second set of conditions. The invention enables, for example, passive addressing of the display and can eliminate gravitational or diffusional motion of the pigment particles over time, thus enhancing the bistable nature of the display.
Thus, threshold addressing allows simple, inexpensive passive addressing techniques to be employed. The inverse electrorheological gating techniques allow the construction of a full color, passively addressed display without the need to address individual capsules. This allows a mixture of capsules to be coated randomly onto a substrate, again enabling the creation of inexpensive displays. All the techniques described allow a significant threshold to be realized, which allows rapid movement of the pigment particles above the threshold, which in turn allows rapid switching times for the display. The use of grafted molecules to provide steric stability to the pigment and floc network particles combined with controlled strength ion pairs instead of physically adsorbed dispersants and charging agents, allows improved control over the conductivity of the display medium in both encapsulated and traditional electrophoretic displays. Since diffusion and gravity are extremely weak forces compared to the yield stresses created, the structured or gelled medium approach also provides a great improvement in the bistability of the displays.